1. Field of the Invention
The present invention relates to an apparatus for and a method of evaluating a multilayer thin film and more particularly to an apparatus for and a method of evaluating a multilayer thin film capable of evaluating the thickness and the boundary state of each layer of the multilayer thin film formed by epitaxial growth of a semiconductor crystal, for example, in a non-destructive and non-contact manner.
2. Description of the Prior Art
Generally, an interference phenomenon of light is used for measuring the thickness of a thin film in a non-destructive and non-contact manner.
For example, a sample having semiconductor thin film layers 2, 3 and 4 formed on a semiconductor substrate 1 is shown in FIG. 1, on the surface of which a predetermined light beam 5 impinges at an incident angle .theta.. Numerals 6. 7, 8 and 9 designate one-dimensional reflected light components on the surfaces of the thin film layers 2, 3 and 4 and the substrate 1, respectively. The thicknesses and refractive indices of the thin film layers 2, 3 and 4 are designated as (d1,n1), (d2,n2) and (d3,n3) respectively and the refractive index of the substrate 1 is designated as ns.
The reflected light components 6, 7, 8 and 9 on the surfaces of the respective thin film layers 2, 3 and 4 and the substrate 1 generate phase differences due to each optical path length and are synthesized to interfere with each other on the surface of the sample. When the i-th thin film layer from the top is defined as the i-th layer (where i is an integer), an optical path difference .delta..sub.i between the reflected light component 6 on the surface of the top layer and the reflected light component in the interface of the i-th layer and the (i+1)-th layer is expressed by the following formula: ##EQU1## Thickness informations of the respective thin film layers 2, 3 and 4 can be obtained by analyzing a spatialgram of a reflected light beam formed by synthesizing the respective reflected light components having the phase differences .delta..sub.i.
In general, a method of evaluating the film thickness from the analysis of an interference fringe of a reflection interference spectrum of the thin film has been conventionally adopted. This method is effective for the film structure consisting of a single layer, however, it cannot be practically used for the film structure consisting of plural layers because it is very difficult to separate and analyze each interference fringe.
Fourier transform infrared spectroscopic method (FTIR method) using Fourier analysis has been proposed as a method of measuring the thickness of &he multilayer thin film in a non destructive and non-contact manner. FIG. 2 is a schematic structural view showing an optical system A of an apparatus for evaluating the multilayer thin film using the FTIR method, and FIG. 3 is a general structural diagram of the apparatus.
As shown in FIGS. 2 and 3, an infrared light beam in a predetermined wave number region is emitted from a light source 10. The wave number region of &he infrared light beam is set according to the crystalline materials constituting the multilayer thin film of a sample for example, at 12000-2000 cm.sup.-1 for AlGaAs series and at 8000-1OOO cm.sup.-1 for InGaAsP series.
The light beam emitted from the light source 10 is transformed into a parallel light beam by an aspherical mirror 12 to be led to a Michelson interferometer 13.
The Michelson interferometer 13 comprises a beam splitter 14 for splitting the incident parallel light beam into two beams: a transmitted light beam and a reflected light beam, a fixed mirror 15 for reflecting the transmitted light beam of the beam splitter 14, a mobile mirror 16 for reflecting the reflected light beam of the beam splitter 14 and a driver 17 for transferring the mobile mirror 16 at a constant speed in the direction shown by the arrow of FIG. 2. The parallel light beam which is incident on the Michelson interferometer 13 is splitted by the beam splitter 14 into two beams: the transmitted light beam and the reflected light beam. After reflected by the fixed mirror 15 and the mobile mirror 16 respectively, the transmitted light beam and the reflected light beam return to the beam splitter 14 again and are synthesized to interfere with each other on the surface thereof. Since the mobile mirror 16 is transferred at a constant speed in the direction shown by the arrow of FIG. 2 by the driver 17, the transmitted light beam and the reflected light beam are synthesized while continuously varying the optical path differences thereof. Thus, the interference light to be synthesized on the beam splitter 14 is the light beam modulated with time according to the constant speed travelling of the mobile mirror 16. The interference light beam is led out toward an optical system 18 for lighting the sample.
The interference light beam led to the optical system 18 is converged on the surface of the sample 11 by an aspherical mirror 19 in order to improve the utilization efficiency of light beam energy. As described above, the light beam reflected by the sample 11 is subject to the interference caused by the film structure of the sample 11 and converged through an aspherical mirror 20 on the light-receiving surface of a photo detector 21.
Thus an interferogram (i.e., a spatialgram including noise) is measured by the photo detector 21. The interferogram measured by the photo detector 21 is subject to Fourier transform by Fourier transform means B to obtain a reflection spectrum. Next, filtering means C filters the reflection spectrum to remove wave number regions having no photometric sensitivity therefrom. The filtered reflection spectrum is subject to reverse Fourier transform by reverse Fourier transform means D to obtain a spatialgram excluding noise.
FIG. 4 shows an example of the spatialgram provided by the use of the multilayer thin film sample of FIG. 1. In FIG. 4, the abscissa indicates a travelling distance of the mobile mirror 16 and the ordinate indicates an interference intensity of the reflected light beam. As shown in FIG. 4, in the spatialgram appear bursts 22-25 which is caused by the mutual intensification of total light due to the interference when the optical path difference by the travelling position of the mobile mirror 16 agrees with the optical path differences of the respective reflected light components indicative of the formula (1). The distances between the respective bursts correspond to the optical path differences of the respective reflected light components. In the example of FIG. 4, each side burst 23, 24 and 25 corresponding to the reflected light components 7, 8 and 9 (in FIG. 1) of the respective layers appears symmetrically, taking as an origin the center burst 22 corresponding to the reflected light component 6 on the surface of the sample (or the thin film layer 2). When the distances from the center burst 22 to the respective side bursts 23, 24 and 25 are designated as L.sub.i (i=1,2,3), the optical path differences .delta..sub.i of the respective reflected light components coincide with 2L.sub.i indicative of the length of the both paths to the mobile mirror 16. Accordingly the following formula can be obtained from the aforesaid formula (1): ##EQU2## where the refractive indices n.sub.j and the incident angle .theta. are known. Therefore the thicknesses d.sub.i of the respective layers can be calculated by the formula (2) if the distances L.sub.i between the bursts are found by using the aforesaid spatialgram.
Evaluating means E of FIG. 3 can thus analyze the waveform of the aforesaid spatialgram to measure the thicknesses of the respective layers of the multilayer thin film. Furthermore, in addition to the thicknesses of the respective layers, boundary states of the respective layers can be evaluated from the steepnesses of the waveforms of the side bursts 23-25, for example.
In the conventional apparatus for evaluating the multilayer thin film constructed as above-mentioned an optical system of a converging system is used as the optical system 18 for lighting the sample as described above. The purposes of the adoption thereof are, by converging the inference light beam emitted from the Michelson interferometer 13 on the surface of the sample to increase the intensity of the detected light to be incident on the photo detector 21, to Improve the SN characteristic of the detection signal thereof, to intend for reducing measurement time, and the like.
Since the optical system 18 of the converging system is adopted, the incident angle .theta. of the light beam 5 projected on the surface of the sample in FIG. 1 is distributed continuously around this value in practice. As a result, variation in transmitted light path of the respective thin film layers 2-4 is generated, and incident wave surfaces are distributed in a certain range. Thus the interference intensity is deteriorated and the burst shapes on the spatialgram are blurred and wide, so that the deterioration in resolution, in measurement accuracy and the like is caused. Particularly in measuring the thin film, because the spatialgram shows a quite smooth intensity distribution with respect to the wave number, a slight change in the intensity distribution due to measurement errors and the like results in the change in the waveform of the spatialgram. As a result, the burst positions are deviated and the adjacent bursts overlap each other, so that the variation in film thickness measured values is caused and the measuring limit thickness of the thin film grows large.
Accordingly the formulas (1) and (2) cannot be used in an intact form. It is necessary to consider the distribution of the incident angle .theta. and deflection characteristics of reflection.
In the conventional apparatus for evaluating the multilayer thin film, the reflection spectrum is transformed into the spatialgram in the reverse Fourier transform means D by cosine reverse Fourier transform by means of a cosine term shown in the following formula (3): ##EQU3## where R(.sigma.): reflected light intensity, f(.sigma.): filtering function, .sigma.: wave number (1/cm), X: distance (cm), and .sigma.s/.theta.e: photometrical wave number limits.
FIG. 5 shows another example of the spatialgram, in which the respective thin film layers 2, 3 and 4 of the sample are 0.503 .mu.m, 0.314 .mu.m and in thickness respectively. As shown in FIG. 5, burst peaks can be seen in the positions corresponding to the interfaces of the respective layers.
As above-mentioned, since the reflection spectrum is subject to the cosine reverse Fourier transform having only the cosine term, the burst waveform which appears on the spatialgram can show a reverse phase having upward/downward burst peaks according to filtering conditions (e.g., the form of the filtering function f(.sigma.) and a filtering wave number region). When the film to be measured is thin, the burst waveforms having upward and downward peaks overlap each other as shown in the spatialgram of FIG. 6, for example. As a result, each peak is swallowed up by a synthesized waveform so that it is difficult to read the peak positions.
In the method of measuring the film thickness by means of the FTIR method, a photometrical wave number range (.sigma.s - .sigma.e) is a major factor determining the thin film measuring limits. In the framework of the photometrical wave number range determined mainly from a photometrical optical system, it is important to read the peak positions from the burst waveforms on the spatialgram. However, in the prior art, the burst waveforms themselves have an unstable factor of the upward/downward phase, which is a factor providing the thin film thickness measurement with a limitation.